Objective: The purposes of this study were: 1) Determination the correlation between gross motor function of children with cerebral palsy and their level in Gross motor function Classification System2) Evaluation and comparison of changes in gross motor function in children with cerebral palsy based on their severity.

Materials & Methods: In this longitudinal and in terventional study 30 children with CP (4-12 years) were recruited from some of Tehran rehabilitation clinics based on pre determined inclusion criteria. then this children based on their level in gross motor classification system were divided non randomly into three groups (mild, moderate and sever). For data collection demographic information questioner, Gross motor Classification system and Gross motor function measure were used. Data were collected before and after intervention. Intervention included two conventional occupational therapy sessions per week and lasted for three months. Data were analyzed by Spearman correlation coefficient, Wilcoxon, Man-whitney and Kruskal Wallis test.

Results : There was inverse correlation between gross motor function level and gross motor function of children in pre & post occasion. in pre test occasion the mean of gross motor function in mild group was significantly higher than moderate & sever group and there was significant difference between moderate and sever group but in the post test occasion there was no significant difference between mean of gross motor function in moderate and mild group and the mean of gross motor function in moderate group was significantly higher than severe group .The most changes in gross motor function were seen in level 3 and 4.

Conclusion: The highest increase in gross motor function after occupational therapy services was obtained by children in level 3 of gross motor classification system.